1. Field of the Invention
The present invention relates to a process for the recovery of methyl cyclosiloxane from silicone scrap material, which includes elastomeric scrap, parings, mold flashes, mold component, extruded profile, RTV mold and off-specification discarded material of silicone polymers hereinafter referred to as `silicone feedstock`. More particularly the present invention relates to a process in which almost pure cyclics are recovered from silicone feedstock without generating further secondary waste. Methyl cyclosiloxanes is used in the manufacture of silicone polymers.
The silicone feedstock that is available can be in different forms e.g., cured, partially cured or uncured silicone in the form of heat cured rubber (HCR); room temperature vulcanising, (both RTV-1 & RTV-2) and liquid silicone rubber (LSR). They may be condensation or addition cured type, filled with reinforcing and/or semi-reinforcing fillers, having no or little commercial utility in their commonly available form or quality.
The difficulty faced by the industry was to recover Methyl cyclosiloxane from filler filled silicone feedstock. Almost all such silicone feedstock contain fumed silica, also known as silicic acid, colloidal silica, millimicron silica etc., as reinforcing fillers. No process is known for recovering cyclics from such filler filled silicone feedstock successfully. In case of filled silicone feedstock, with mainly fumed silica, corresponding metal silicates are produced. Metal silicates find multifarious use in industrial process and can be disposed of.
First the silicone feedstock is liquified in presence of anionic surfactants/amines and/or mineral acids as catalyst where a volatile solvent is used as media. When fumed silica is present in the silicone feedstock, it is removed as a silicate from the liquid mixture by decantation or mechanical separation. After separation of such filler, the volatile solvent is removed by stripping at low pressure and Methyl polysiloxane is obtained as a residue. Methyl polysiloxane can be converted to a mixture of cyclics by cracking with alkali metal hydroxide or mineral acid at high yields which is as high as over 90%.
2. Desription of the Related Art
The use of silicone and silicone containing materials are increasing world-wide and a significant quantity of such silicone feedstock are generated during manufacture and processing of such silicone products. The known process for the utilization of silicone feedstock lacks versatility and is applicable to a particular type of silicone feedstock. In some cases, recovery was very low.
U.S. Pat. No. 4,111,973 issued to Bluestein relates to an improved process for obtaining better yield and purity of fluoroalkyl cyclotrisiloxane in a cracking reaction of diorgano polysiloxane using an effective amount of aliphatic alcohol as a stabilising agent with addition an effective amount of cracking catalyst.
This process is specifically directed to the recovery of pure fluoroalkyl cyclosiloxane from a mixture of diorganopolysiloxanes. The text of this patent is silent as to the effects on the gel mass when small amount of trifunctional groups, is present as impurity. Therefore, this process is only applicable to pure difunctional alkyl polysiloxane.
U.S. Pat. No. 4,764,631 issued to Halm et al provides a method for preparing a product cyclodiorganopolysiloxalle via the vapour phase re-arrangement of other cyclopolydiorganosiloxane or mixtures thereof. This process is also applies to pure difunctional cyclopolydiorganosiloxane and the process is applicable to silicone feedstock which are in the form of volatile cyclics only.
U.S. Pat. No. 2,860,152 issued to Fletcher teaches a method of producing cyclic diorganosiloxanes having a composition different from starting organo polysiloxane. In this process the diorgano polysiloxane and an inert solvent having b.p. more than 250.degree. C. were used. At least 20% by wt. of the siloxane was used as solvent. Temperature and pressure in reaction zone were maintained in a manner such that in presence of alkali catalyst, only cyclics were available from the reaction mixture while the solvent remained non-volatile under those conditions. The inert solvent shifts the polymer/cyclic equilibrium of the reaction more towards the cyclics and with more cyclics in the reactor, lesser the tendency of the reaction mass to gel. Therefore, at least 20% solvent is required to delay the gelation of the reaction mass. However, at the end of run, when the reaction mass gets gelled, the entire reaction mass is discharged as the siloxane/solvent cannot be separated.
Thus, the process does not completely resolve the problem of gelation and therefore recovery is poor.
U.S. Pat. No. 5,420,325 issued to John S Razzano teaches a method of producing cyclics where an effective amount of high boiling alcohol is used in the liquid siloxane hydrolysate. The high boiling alcohol allows the removal of trifunctional species in an efficient manner and completely eliminates formation of a gel. The siloxanes and the alcohol in the residue can be recovered and reused. Thus, Razzano has resolved the problem of gelation by cracking a diorganopolysiloxane containing a portion of trifunctional group to a mixed cyclosiloxane. Razzano has not, however, taught as to how to recover the remaining siloxane from alcohol.
In the aforesaid literature the focus has been on the cracking of a liquid organopolysiloxane containing a portion of trifunctional group but there is no teaching with regard to silicone feedstock filled with fillers, specially fumed silica. These processes can function only in case of liquid methyl polysiloxane which is free of fillers.
U.S. Pat. No. 5,110,972 issued to Tremco Incorporated teaches a method where silicone scrap is dissolved in high boiling solvent (b.p. greater than cyclics) and sulphuric acid. The sulphuric acid is then neutralized with KOH and with additional of KOH methyl polysiloxane is cracked to cyclics at 115.degree. C. under reduced pressure. Again, the said process can only be applied to unfilled feedstock.
There is no teaching in this prior art as to how the residual silicone is separated from filler and solvent rich residue. Thus this process also does not aim at solving the problem of recovery of methyl polysiloxane from filler filled scrap.
Chinese Patent No. CN 1086518A describes a method of manufacturing organocyclic silicone compounds by pyrolysis of silicone rubber under atmospheric pressure. This patent uses silicone rubber as a raw material. After washing and breaking into small pieces, the rubber is mixed with organo cyclosiloxane compounds and reacted in presence of KOH as a catalyst under normal atmospheric pressure and at 200-500.degree. C. This process produces unfavourable results at 200-500.degree. C. in presence of KOH because demethylation of dimethylpolysiloxane occurs rapidly, leading to an explosion.
German Patent No. DE 4126319 A1 relates to a process for the recovery of silicone cyclic from silicone rubber. In this invention, silicone rubber is pyrolyzed at 550-600.degree. C. under vacuum and/or inert atmosphere. Major component in the outlet is Hexamethylcyclotrisiloxane (D3). In this paper the process and apparatus used are not clear. Even if practicable, the capital cost of such process using rotating pipe oven will be extremely high because of (a) non-feasibility of high temperature rotary seal; (b) inert gas blanketing of seal face against accidental leak; (c) complex anti-fouling condensing of D3, a major condensate which sublimes at 64.degree. C.; (d) hazardous in combination with oxygen at 600.degree. C. as this causes rapid oxidation; (e) non-continuous operation due to intermittent cleaning of reactor walls of complex crust formed; (f) progress of pyrolysis hindered by crust formed around the rubber; (g) long retention time for complete pyrolysis. With these disadvantages this process has many practical drawbacks. None of the prior art therefore teaches recovery of siloxane from a non-specific silicone feedstock in such a way that process will be free from waste generation and will be environment friendly.